For the projection copying of masks on a semiconductor substrate in the manufacture of integrated circuitries it is customary to align masks and semiconductor substrate, before the imaging of circuit patterns, relatively to each other in the image plane or the object plane, respectively.
For this purpose it has become known, for example, to allocate to the mask an alignment pattern consisting of a transparent window and to the semiconductor substrate an associated line mark. The window and the line mark are imaged in one another by way of the projection optic and are scanned by an evaluating device with the aid of a rotating prism. This evaluating device measures the time intervals between the arrival of the window edges and the line mark and emits control signals to servomotors for the displacement of the semiconductor substrate until the line mark lies at the center of the imaged window of the mask.
Besides this alignment process in the XY coordinates of the image or object plane it is also necessary with optical copying systems to provide for an exact focusing of the optical system, i.e. to arrange the image and object surfaces--given in the described instance of utilization by the masking layer on the one hand and by a photosensitive layer on the substrate surface on the other hand--exactly in the image and object planes.
For an automatic execution of this so-called Z adjustment it has become known, e.g. from German patent No. 24 27 323, to displace a focused beam in the object and/or the image plane along a circular path and to monitor automatically the state of focusing of the beam. Upon the occurrence of defocusings, site-dependent signals are generated for the correction of alignment errors. With this process it is disadvantageous that for the determination and correction of focusing errors a separate adjustment operation as well as relatively expensive equipment is required.
It has further become known from German open specification No. 26 33 297 to utilize adjustment marks, used for the XY adjustment, also for the Z adjustment. For this purpose, light rays reflected by the adjustment marks are directed onto photosensitive elements and the projection objective is shifted along the optical axis until the reflected light received by the photosensitive elements is at a minimum. The drawback of that process, however, is that only relatively large-area adjustment marks can be used and the achieved accuracy is often inadequate.